The present invention relates generally to continuous wave radars and, more particularly, to test circuitry and a related method of testing to evaluate the performance of the radar system.
Continuous wave (CW) radars have the capability to determine target distance and relative velocity, and thus are used for many different purposes. Either one, or both, of these measurement parameters can be determined depending upon the application. The CW radar is especially adaptable to the detection and measurement of targets at relatively short distances, with some examples being automobile speed determination for traffic enforcement, ground proximity warning radar for airborne use, and intrusion detection in both permanent and temporary installations.
A fundamental characteristic of the CW radar is that transmission and reception occur simultaneously, in contrast to a pulse radar that transmits first and then receives target returns after the transmitter shuts off. In the CW radar the difference in frequency between the transmitted and received signals is analyzed to detect the target and determine parameters of interest. Many variations of the CW radar exists, including frequency modulated CW (FM-CW), frequency shift keying (FSK-CW), pseudo-random coded (PN-CW), as well as the basic fixed frequency CW radar. The modulation characteristics of the transmitted signal are tailored to meet the application requirements.
One modulation example is the transmission of a continuous wave at a fixed frequency in a simple radar to determine the velocity of approaching or receding automobile targets. The Doppler shift in the reflected signal is processed to determine the target""s relative velocity, but no determination of range is possible. Another example involves continuous transmission of a triangularly modulated signal made up of positive and negative, highly linear changes of frequency with time. The slope of the transmitted signal will determine how the frequency difference due to distance is combined with the difference produced by any relative velocity. The returns from targets during the positive slope are compared with returns during the negative slope to allow determination of both distance and relative velocity of the target.
In many applications, it is highly desirable that the CW radar be small in size, relatively low cost, and highly reliable during operation. A radar of small size normally operates in the microwave or millimeter wave portion of the electromagnetic spectrum. The radar should also be designed to be placed into operation with a minimum of test and adjustment. What is needed is a means to conduct a self-test of all major subsystems of the radar, both at first activation and periodically during operation without the need for external test equipment. This means should be integral to the radar, simple to operate, and not add significant cost.
Prior art examples of radar self-test apparatus and methods having some similarity to the present invention include Cherry, et al., U.S. Pat. No. 5,432,516, This patent describes a means to extract a sample of the signal being transmitted, a switch coupled to the sampling means and to a delay unit that is in turn coupled to an antenna. This antenna is positioned in such a manner that the delayed signal is coupled into the field of radiation of the main antenna used for transmission and for receiving signals reflected from targets. When a test of the overall radar performance is undertaken, the switch is closed and the delayed signal enters the main antenna to be processed in the same manner as a target return. An apparent target, that appears at the appropriate amplitude and range corresponding to the delay provided by the delay unit, is an indication that all radar subsystems are functioning properly. The test target appears at a fixed range determined by the characteristics of the delay unit, no provision is made to vary the range radar response at different ranges. The delay unit used in this design often requires relatively expensive components and significant volume, and thus adds cost to the radar. The present invention does not require such a delay unit, and allows the range of the test target to be varied.
Another example of prior art is provided by Broxon, et al., U.S. Pat. No. 5,886,663. Disclosed is the use of a modulation diode positioned within the field of radiation of the radar antenna. This diode is driven to conducting and non-conducting states by a low frequency signal generator that is activated when a test of the radar is performed. When conducting, the diode changes the voltage standing wave ratio of the radar antenna, thus reducing its gain. Targets external to the radar and within the field of radiation reflect the transmitted signal back to the antenna where it is received and processed. When the diode is conducting, the reduced gain of the antenna results in a reduction of the amplitude of the received signal. A comparison of the processed target signals with the diode conducting and non-conducting allow determination of the status of the radar subsystems. The test requires the existence of a target or targets of opportunity within the field of radiation of the radar antenna.
These prior art examples either require complex auxiliary circuitry to perform the needed system testing and thus add significant expense to the radar, or in the case of Broxon, et al., the existence of external targets are required. Thus, a need exists for a highly efficient and economical self-test capability for use in CW radars that is capable of determining the overall operability of the radar, including its antenna, interconnecting cables, and all electronic subsystems. This self-test capability should be independent of the external environment of the radar and function either upon command or automatically on a periodic basis. For these reasons, as well as other reasons, there is a need for the present invention.
Accordingly, the present invention provides a new and improved method for evaluating the performance of a CW radar. It is an advantage of the present invention that only a small number of relatively inexpensive components are required in its fabrication. It is also an advantage of the present invention that the testing process includes collecting a small sample of the radar""s radiated beam and imposing modulation on this sample by alternately coupling the line conducting the sample to a dissipative load or to a reflective short. The result is substantial amplitude modulation imposed upon the sample to form a test signal with frequency content related to the rate of switching between the two terminations. The test signal is re-radiated in such a manner that a portion is received by the radar""s antenna. Within the radar, the test signal is amplified by the receiver, processed by the frequency analyzer and presented on the display as a test target. Therefore; every major circuit within the radar is exercised by the test method, and any circuit failure will be indicated by the diminished or non-existence of a displayed test target. It is a further advantage of the present invention that the test target can be varied in range throughout the radar""s range limits. Any range dependent failure in the radar""s circuitry can thus be identified. Still other advantages, aspects, and embodiments of the invention will become apparent by reading the detailed description that follows, and by referencing the attached drawings.